1. Field of the Invention
The present invention relates to a device for digitally slaving the position of a moving part.
2. Description of the Related Art
Digitally slaving the position of a moving part is useful in the field of directional radio equipment, the said moving part being, for example, a plunger whose position of varying insertion through a longitudinal slot made in a rectangular waveguide determines the variation in a characteristic quantity of the electric field propagating inside the guide. On account of the essentially resistive nature of the plunger, this produces an attenuator for a rectangular waveguide.
A digital slaving device generally includes:
a digital sensor which is intended to measure the actual position of the moving part and delivers a digital effective position value, PA1 drive means comprising an electric motor for driving the moving part into a setpoint position defined by a digital setpoint value, PA1 and a digital control unit delivering at least one signal for controlling a circuit for supplying the motor as a function of the discrepancy between the said setpoint value and the said actual value. PA1 a high speed is a speed of rotation of the motor which is large enough for it to be possible, after the motor supply has been cut, for the motor to drive the moving element through its inertia beyond the position which it occupied when the supply was cut, which may result in a positional error for this element; PA1 conversely, a slow speed is such that, when the motor supply is cut, the motor stops moving without a significant delay, so that the moving element remains in the position which it occupied at the moment when the supply was cut.
The digital sensor comprises, for example, an encoder of the Gray type formed by a set of discontinuous conductive tracks, as well as a transcoder for converting the value supplied by this encoder into a digital value in binary code.
The digital sensor has a measuring interval between two successive values which is equal to 1, as can be seen for example in FIG. 1, which shows a digital encoder of the Gray type including three concentric discontinuous annular conductive tracks P1, P2, P3. These tracks are engraved on a dielectric substrate SD and are all connected to a DC supply voltage. Furthermore, as can be seen in FIG. 2 which is a three-dimensional view of an encoder of this type, they also have three elastically mounted contact terminals, BC1, BC2 and BC3 respectively, rubbing against them. The signals which these contact terminals deliver determine the value, in Gray code, corresponding to the angular position of the encoder when it is rotated about its axis AR.
The encoder which is described is an encoder having eight separate angular positions distributed regularly over 360.degree.. The measuring interval can then be expressed as the angle of the annular sector within which the terminals BC1, BC2 and BC3 remain individually subjected to a constant potential and therefore indicate a constant angular position. This interval is 1=45.degree. with the three-track (and therefore eight-position) encoder in FIG. 1.
Furthermore, this figure schematically uses a bold line, referenced BC and lying on a radius of the circles constituting the generatrices of the annular tracks P1 to P3, to represent the relative angular position of the contact terminals BC1 to BC3 on the tracks.
The drive means may also comprise a reducing gear arranged between the output shaft of the motor and the moving part. The encoder is linked in rotation with the output shaft of the motor or some element of the reducing gear.
If the speed of rotation of the motor is slow enough in relation to the resistive torque applied to it, the movement of the driven part is stopped as soon as the difference between the digital values for setpoint position and actual position becomes zero, and the supply to the motor is consequently cut.
The radius BC then substantially coincides with the boundary between the setpoint sector sc and the preceding adjacent sector, this being defined with reference to the direction of movement of the tracks relative to the contact terminals. However, the direction of rotation of the motor depends on the setpoint position relative to the position previously occupied by the moving element, that is to say, furthermore, the sign of the difference between the setpoint value and the actual value. Indeed, the slaving is intended to make this difference zero (this is also referred to as convergence between the actual position and the setpoint position). Therefore, depending on the direction of rotation of the motor, the said sector will be one or other of the two sectors SA1 or SA2 adjacent to the setpoint sector SC.
Between these two situations, which correspond to a unique balance position for the digital slaving, there is an error on the actual position of the driven moving element, and this error is proportional to the measuring step 1 of the digital sensor. This error does, of course, decrease as the measuring interval 1 becomes smaller, that is to say when the encoder has a large number of separate positions. In addition, it can be further reduced by at best a factor of N, where N is the reducing gear ratio, if the encoder is appropriately connected with this reducing gear. Nevertheless, there are applications in which the effects of this error, even when reduced as far as possible, are still unacceptable. This is the case for the application envisaged above, in which the height to which the plunger is inserted into the guide is critical. Therefore, there is a need for a simple and effective way of eliminating the effects of the measuring interval of the digital position sensor.